Add AGENTS.md for AI coding agents

[0x46616c6b]

Type of Change

• Documentation

Description

Adds an AGENTS.md file with compact, repo-specific guidance for AI coding agents (OpenCode, Copilot, Cursor, etc.) working in this repository.

The file covers the conventions most likely to be missed by an agent without prior context:

• Action pinning -- full SHA with version comment, never tag refs
• Template naming -- template_ prefix convention
• Self-referencing -- @main for internal workflow calls
• GitHub App auth pattern -- the USING_APP_CREDENTIALS dual-auth idiom
• YAML style -- non-obvious .yamllint settings
• Release automation -- don't manually edit README version refs

Checklist

• Add relevant labels (for example type of change or patch/minor/major)
• Make sure not to introduce some mistakes
• Update documentation
• Review the Contributing Guideline and sign CLA
• Reference relevant issue(s) and close them after merging

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_{The changes and the PR were generated by OpenCode.}